1. Technical Field
This invention relates to streamlined bodies with airfoil shaped cross sections.
2. Background Art
Bodies with a streamlined shape are used within a moving fluid for several reasons, including, but not limited to one or more of the following: to reduce losses as a result of drag; to minimize turbulence created downstream; and to create or increase lift (such as in the case of an airplane wing). Streamlined bodies having airfoil shaped cross-sections are the most common.
In this specification and the appended claims, a body having an xe2x80x9cairfoilxe2x80x9d shaped cross-section means any elongated, streamlined body that, when placed within a fluid moving downstream relative to the body, the body will have a pressure surface and a suction surface; and those surfaces join together to form a leading edge region and a trailing edge region. The surface which is the pressure surface and the surface which is the suction surface is determined by the angle of incidence of the bulk flow on the body. Typically, a body is designed to have one of the surfaces always perform as a suction surface, and the other always perform as a pressure surface; however there are some applications for which the roles of these surfaces become reversed during a portion of the operating regime.
Much work has been done over the years to increase the effectiveness of bodies having airfoil shaped cross-sections. For example, considerable effort has gone into delaying boundary layer separation from the suction surface for the purpose of reducing drag, to increase lift, to delay the onset of stall (i.e. to accommodate higher angles of incidence of the flow against the leading edge), and for generally giving the airfoil the ability to perform better over a wider range of flow conditions.
One common and successful method used to achieve these goals has been slot blowing. Slot blowing according to the prior art is the injection of pressurized fluid from within the body along the suction surface at a point just upstream of where separation is normally expected to occur. The fluid is injected from appropriately shaped slots oriented to direct the fluid in a downstream direction substantially parallel to or at a low angle to the surface. Slot blowing adds momentum to the near-wall flow, increasing the boundary layer near-wall energy thereby allowing the boundary layer to remain attached to the surface beyond the point where separation would have occurred without the blowing. Prior art slot blowing is discussed in more detail in the paper AIAA 98-0214, Oscillatory Control of Separation at High Reynolds Numbers, by A. Seifert and L. G. Pack.
A drawback of downstream slot blowing is the relatively high rate of fluid injection needed to have the desired impact on boundary layer separation. The use of pressurized fluid for the downstream slot blowing entails a cost, since energy needs to be expended to pressurize the fluid. As a result, for some applications, downstream slot blowing is impractical or must be limited in order that the costs of blowing do not outweigh the benefits.
According to the present invention, a streamlined body having a leading edge region includes passageways within the body for directing pressurized fluid from within the body over the external surface of the body at a shallow angle with respect to the external surface and toward the leading edge region. According to an exemplary embodiment of the present invention, a streamlined body has a surface that is the pressure surface during at least a portion of its operation in its intended environment. Passageways within the body have their outlets at the pressure surface and are located and oriented to inject pressurized fluid upstream over the pressure surface at a shallow angle to the pressure surface toward the leading edge region. The passageway outlets are sufficiently close to the leading edge region and the angle of injection is sufficiently shallow that substantially all the injected fluid travels along the pressure surface and around the leading edge region to the suction surface, energizing the boundary layer on the suction surface.
Injecting the flow in an upstream direction over the pressure surface allows several beneficial phenomena to take place, which do not occur with prior art downstream slot blowing over the suction surface. First, the shear layer between the injected flow and the freestream flow has a longer development path than with downstream slot blowing before reaching the point on the suction surface where separation would normally occur. In other words, in the present invention, for the injected fluid to travel from the passageway outlets to the aforementioned normally occurring separation point, it must travel further than fluid injected downstream in accordance with the prior art. This gives energetic, large-scale turbulence structures more distance to develop. Additional energetic turbulence provides enhanced entrainment of the freestream flow on the suction surface, thereby delaying (to a point further downstream) or eliminating separation.
Second, the injected fluid accelerates around the leading edge region, thereby increasing its momentum and kinetic energy, which enables the boundary layer to remain attached to the suction surface for a longer distance than it would without this aceleration.
Third, the injected fluid, as it travels around the leading edge region of the body, creates a xe2x80x9cvirtual shapexe2x80x9d around the leading edge region, effectively making the leading edge region operate as though it were thicker. Thickening the leading edge region of an airfoil is a well-known approach for reducing, delaying or eliminating the boundary layer separation and stall experienced by an airfoil at higher angles of attack. The present invention provides the benefits of a thicker leading edge region without actually adding thickness and weight. In the alternative, a streamlined body incorporating the features of the present invention may be made thinner without incurring separation related performance losses.
As a result of the foregoing phenomena, the present invention, using a lower rate of fluid injection (usually accompanied by lower pressure, as well) provides benefits similar to and even better than those achieved with prior art downstream slot blowing. Alternatively, significantly improved performance (e.g., greater lift, greater stall margin, and less drag) may be obtained with the present invention using the same rates of fluid injection as used by the prior art.
In one embodiment of the present invention, a streamlined body having an airfoil shaped cross-section has one or more passageways therein communicating with a source of pressurized fluid. The passageways have their outlets on the pressure surface a short distance downstream from the leading edge line; and the passageways are oriented to direct the pressurized fluid toward the leading edge line at a shallow angle with respect to the pressure surface at the passageway outlet.
The xe2x80x9cleading edge linexe2x80x9d, as that phrase is used herein, is the span-wise locus of points formed by the intersection of the mean camber lines of the airfoil shaped cross-sections with the leading edge region. To optimize the benefits of the invention, the flow losses within the passageways should be minimized and the minimum amount of pressurized fluid required to obtain a desired result should be used. For this reason the passageways are preferably slots, elongated in the direction of the leading edge line to provide wide coverage of the injected fluid over the length of the leading edge region, and ultimately over the suction surface. Elongated slots have lower flow losses than, for example, a series of holes of circular cross-section providing the same mass flow rate over the same length of leading edge region.
In order to realize the benefits of the present invention the passageway outlets cannot be so far downstream of the leading edge line that an excessive amount of the injected fluid is unable to remain attached to the pressure surface as it moves toward and around the leading edge region to the suction surface. As the passageway outlets are moved further from the leading edge line, larger quantities of blowing fluid and/or higher pressure fluid may be required to compensate for fluid lost to the free stream or bulk fluid flow. At some distance downstream it will become impractical and/or impossible to have a sufficient amount of the injected fluid remain attached to the pressure surface until it travels around past the leading edge line to the suction surface. Generally, a good slot outlet location is near the stagnation point. This may turn out to be just in front of or just behind where the stagnation point would be located without slot blowing. Upon the initiation of slot blowing toward the leading edge line, the stagnation point will move (from its location without blowing) whenever separation on the suction surface is delayed. In some circumstances, it may move from in front of the slot outlet to behind the slot outlet.
If slots are used for the passageways, they may be formed in any number of ways. The simplest slot is perhaps a straight passageway of rectangular cross-section through the wall of the streamlined body, which is normally a hollow body. A vector representing the direction of flow from the slot should form a shallow angle with respect to the surface. Any number of passageway designs may be used to make that angle as small as possible. Several such designs are described below in the xe2x80x9cBest Mode for Carrying Out the Inventionxe2x80x9d.
Without intending to limit the same, the present invention is particularly useful in connection with compressor and turbine rotor blades and fixed turning vanes, helicopter blades, aircraft wings, nacelles, aircraft and engine struts, and aerodynamically contoured control surfaces, such as aircraft horizontal and vertical stabilizers and flaps. All of these are considered streamlined bodies having airfoil shaped cross-sections since, during use, the external surfaces become pressure and suction surfaces. Nacelles, for example, such as those surrounding aircraft engines, are typically defined by rotating a planar airfoil shape around a downstream extending axis. Rather than to create lift, nacelles are designed to reduce drag and to minimize or reduce distortion of the air entering the engine under a variety of operating conditions. The present invention is particularly useful for reducing or minimizing distortion during high angles of attack and when there are cross winds, as is explained more fully below.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.